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Edge Triggered 555 Monostable Schematic

Edge-Triggered 555 Monostable Multivibrator

While the 555 oscillator IC is a very versatile device, it can also be quite tricky and solutions are not always obvious. Such is the case with this edge-triggered 555 monostable circuit. By differentiating the trigger signal, the monostable multivibrator times out normally regardless of the trigger signal length.

555 Schematic

Edge Triggered 555 Monostable Schematic

More than a capacitor

The classic differentiator circuit consists of a capacitor and resistor. In this way, the positive change in the trigger signal is coupled to the 555 via C2. When the trigger signal eventually resets, D2 is required to prevent the change in voltage from creating a substantial negative spike. While a negative transient of this nature is unlikely to cause damage, it could well cause temporary IC malfunction.

R-C timing circuit

C1 and the series resistance of R1 to R3 affect the timeout function. Circuit values may be selected per application requirements. This part of the circuit is fairly typical.

Driving the relay

The 555 output is virtually always in the incorrect polarity. However in this case, the relay is working against the positive 12V rail so its operation is effectively inverted. Note the clamp diode that is across the relay coil.

Oscillographs

 Oscillographs

Photos

About the test circuit

To generate a trigger signal, I used a miniature pushbutton that works against Vcc. The relay contacts are connected to drive an LED.

9 Comments

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  • aerowerks

    on the schematic, you have “trigger” connected to pin6 (threshold) rather than pin2 (trigger) is that intentional or are the pins mislabeled?

    • Jim Keith

      When this device was created, marketing needed catchy terms and so labeled the pins with “trigger” and “threshold.” A more descriptive label would have been “upper threshold” and “lower threshold.” The trigger input in this case is pin 6 –the schematic is correct –sorry to create confusion.

  • forcedfx

    What does diode D1 do in the timing circuit?

    • Jim Keith

      In your case, D1 is not required.

    • forcedfx

      If I only need the output to fire once can I eliminate D1? My input pulse would last several hours but I only want the 555 to output a single 10ms long pulse once the input pulse goes high. I believe this circuit design fits my needs perfectly.

    • Jim Keith

      D2 allows the timer to reset quickly after the 555 times out, otherwise it would not be ready for a subsequent pulse until after a similar time delay. R2 is a potentiometer that is wired as a rheostat — in this case, the top connection does not need to be connected, but is generally done as a matter of convention.

    • forcedfx

      Also, forgive my stupidity but is R2 a rheostat?

  • francisco

    wonderfull assemblie congratulation by your site about 555 sorry to my english that iam a brasilian man and do not understand more about your native language please helpme in words

    • Dan OConnell

      Very good explanation of the circuit operation so that it might be applied in specific applications. Great building block!

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